1. Field of the Invention
The present invention relates to new catalysts for the production of alkenes by selective, partial oxidation of the corresponding alkane and to methods of producing such catalysts and methods of using the same. More particularly, this invention relates to tungsten or manganese-based catalysts for the selective, partial oxidation of alkanes to the corresponding value added product, such as ethylene and acetic acid, with high selectivity, depending on the type of the metal oxide catalyst used in the process.
2. Description of the Related Art
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Many catalysts have been proposed for the activation of light alkane hydrocarbons in oxidation and oxidative dehydrogenation reactions. Some of these catalytic systems make use of one or more catalysts in order to maximize the yield of value added product, e.g., acetic acid via oxidative dehydrogenation of ethane. E. M. Thorsteinson, T. P. Wilson and F. G. Young, Journal of Catalysis, 2:116-132 (1970) were the first to report the use of a molybdenum and vanadium-based mixed metal oxide catalyst for the production of ethane to ethylene. Several other publications have described the use of different catalytic systems for such reactions, including U.S. Pat. Nos. 5,162,578, 4,524,236, 4,568,790, 4,250,346, 5,153,162, 5,907,566, 4,849,003, 4,596,787, 4,339,355, and 4,148,759; European patent application nos. 0294845, 0480594, 0407091, 0518548, and 0627401; WO 9913980; WO 9805620; and U.S. application Ser. Nos. 09/107,115, 09/219,702, 08/997,913, 09/107,046, and 09/085,347.
Due to the great industrial importance of oxygenated hydrocarbons and dehydrogenated products, even a slight improvement in the redox behavior of the metal oxide catalyst, responsible for the activation and product selectivity, can impact tremendously on the catalyst""s performance and strength. Ultimately, such improvements can have a remarkable commercial and economic impact on the process. Therefore, it would be desirable to produce a metal oxide catalyst having improved or modified redox properties which can achieve the goals of high product selectivity or activity.
The present invention provides a method for the selective oxidation of lower alkanes, e.g., ethane, with molecular oxygen to yield the corresponding carboxylic acid and/or olefin, e.g., acetic acid and ethylene, at relatively high selectivity and productivity. The process is carried out at temperatures of 150xc2x0 C. to 450xc2x0 C. and pressures of 1-50 bar. The method is achieved using catalyst compositions containing mixed metal oxides.
The catalyst compositions of the present invention include compositions of the formula:
MoaVbAlcXdYeOz 
wherein:
X is at least one element selected from the group consisting of W and Mn;
Y is at least one element selected from the group consisting of Pd, Sb, Ca, P, Ga, Ge, Si, Mg, Nb, and K;
a is 1;
b is 0.01 to 0.9;
c is  greater than 0 to 0.2;
d is  greater than 0 to 0.5;
e is  greater than 0 to 0.5; and
z is an integer representing the number of oxygen atoms required to satisfy the valency of Mo, V, Al, X, and Y. The catalysts are preferably produced using the methods disclosed herein.
One aspect of the invention relates to a catalyst for the production of olefins and carboxylic acids from lower alkanes via a selective, partial oxidation. In a preferred embodiment, the method of the present invention provides a means for the selective partial oxidation of ethane to yield acetic acid and ethylene.
The catalyst compositions of the present invention comprise compositions of the formula:
MoaVbAlcXdYeOz 
wherein:
X is at least one element selected from the group consisting of W and Mn;
Y is at least one element selected from the group consisting of Pd, Sb, Ca, P, Ga, Ge, Si, Mg, Nb, and K;
a is 1;
b is 0.01 to 0.9;
c is  greater than 0 to 0.2;
d is  greater than 0 to 0.5;
e is  greater than 0 to 0.5; and
z is an integer representing the number of oxygen atoms required to satisfy the valence of Mo, V, Al, X, and Y. The catalysts of the present invention can be used with or without a support. The choice of the individual elements contained in the catalyst composition as well as the specific procedures followed in preparing the catalyst can have a significant impact on the performance of a catalyst.
Preferably, the catalyst is prepared from a solution of soluble compounds (salts, complexes, or other compounds) of each of the metals. The solution is preferably an aqueous system having a pH of 1 to 10, and more preferably a pH of 1 to 7, and it is maintained at a temperature of about 30xc2x0 C. to about 100xc2x0 C. After the elements are combined in solution, water is removed by filtration, and the catalyst is dried, e.g., in an oven at a temperature from 100xc2x0 C. to 130xc2x0 C. The dried catalyst is calcined by heating to a temperature of about 250xc2x0 C. to about 600xc2x0 C., preferably about 250xc2x0 C. to about 450xc2x0 C., in air or oxygen for about one hour to about 16 hours to yield the desired catalyst composition.
Suitable supports for the catalyst include alumina, silica, titania, zirconia, zeolites, silicon carbide, molybdenum carbide, molecular sieves and other microporous/nonporous materials, and mixtures thereof. Support materials can be pretreated with acids, such as HCl, HNO3, H2SO4, per acids or heteropoly acids, and alkali bases. When used with a support, the composition usually comprises from about 5% to 50% by weight catalyst, with the remainder being the support material.
Preferably, molybdenum is introduced into the solution in the form of an ammonium salt, such as ammonium paramolybdate, or as an organic acid salt of molybdenum, such as an acetate, oxalate, mandelate, or glycolate. Some other partially water soluble molybdenum compounds which may be used to prepare the catalyst compositions of the present invention include molybdenum oxides, molybdic acid, and chlorides of molybdenum. Preferably, vanadium, aluminum, gallium, silicon, germanium, antimony, phosphorous, niobium, potassium, magnesium, palladium, tungsten, and manganese are introduced into the catalyst slurry as salts or acids, including but not limited to oxides, hydrate oxides, acetates, chlorides, nitrate acetates, oxalates, oxides, and tartrates.
The present method may be used to oxidize lower alkanes, e.g., C2-C8 alkanes, preferably ethane, propane, and n-butane, as well as alpha-beta unsaturated aliphatic aldehydes. In a preferred embodiment, the starting material is ethane. The starting material(s) may be in the fluid or gas phase. If the starting material(s) is in the fluid phase, the catalyst may convert the reactant(s) to one or more fluid products. The starting material(s) may also be supplied in a gas stream, which contains at least five volume percent of ethane or a mixture of ethane and ethylene. The gas stream can also contain minor amounts of C3-C4 alkanes and alkenes, with the proviso that the gas stream contain less than five volume percent of each. The gas stream can also contain major amounts, i.e., more than five volume percent, of nitrogen, carbon dioxide, and steam.
The reaction mixture used in carrying out the process is generally a gaseous mixture of 0.1 to 99 mol % ethane, 0.1 to 99 mol % molecular oxygen, either as pure oxygen or air, and zero to 50 mol % steam. In a preferred embodiment, the feed mixture contains 0.1-50% by volume molecular oxygen. Further, water may be added as a reaction diluent and as a heat moderator for the reaction. Water added as a co-feed in this way can also act as a desorption accelerator of the reaction product in the vapor phase oxidation reaction or to mask the sites responsible for total oxidation resulting in an increased yield of acetic acid. The amount of oxygen present may be equal to or less than a stoichiometric amount of oxygen in relation to the amount of hydrocarbons in the feed.
The gaseous mixture is generally introduced into the reaction zone at a temperature of about 150xc2x0 C. to about 450xc2x0 C., and preferably 200xc2x0 C. to 300xc2x0 C. The reaction zone generally has a pressure of 1 to 50 bar, and preferably 1 to 30 bar, a contact time between the reaction mixture and the catalyst of about 0.01 seconds to 100 seconds, preferably 0.1 seconds to 50 seconds, and most preferably 0.1-10 seconds, and a space hourly velocity of about 50 to about 50,000 hxe2x88x921, preferably 100 to 10,000 hxe2x88x921, and most preferably 200 to 3,000 hxe2x88x921. The process is generally carried out in a single stage in a fixed bed or fluidized bed reactor with all the oxygen and reactants being supplied as a single feed. Non-reacted starting materials can be recycled. However, multiple stage additions of oxygen to the reactor with intermediate hydrocarbon feed can also be used. This may improve the productivity of the process and avoid a potentially hazardous condition due to explosion limits of the mixture of hydrocarbons and oxidants.
The catalyst of the invention is not limited to use in the oxydehydrogenation of ethane to acetic acid. The catalyst of the present invention may also be used (1) to oxidize alpha-beta unsaturated aliphatic aldehydes in the vapor phase with molecular oxygen to produce the corresponding alpha-beta unsaturated carboxylic acids, (2) to oxidize C3 alkanes or alkenes to the corresponding acids, and (3) to ammoxidize alkanes and/or alkenes. In a preferred embodiment, the method is used to selectively oxidize ethane, with little or no carbon monoxide as a side product. In one embodiment, the maximum amount of carbon monoxide produced is about 2% based on percent selectivity. Further, the method yields product at a selectivity preferably of at least 80%, more preferably at least 82%, and even more preferably at least 90%, and a conversion rate preferably of at least 7%.
The following examples are intended to be illustrative of this invention. They are, of course, not to be taken to in any way limit the scope of this invention. Numerous changes and modifications can be made with respect to the invention without departing from the spirit or scope of the present invention.